Genome rearrangement can be directly responsible for oncogenic transformation, and its structural requirements need to be investigated. A specific site of genomic rearrangement has now been correlated with undermethylation in the chicken genome. Recombination involves a short, tandemly repeating region contained within a longer repeated sequence element and the recombination event generates a polar deletion. DNA sequencing of cloned DNA segments provides a complete description of this undermethylated recombination site. Another way methylation might affect oncogenic transformation is through repression of incoming (virally carried) oncogenes; cytosine methylation apparently inhibits transcription. The specific requirements for de novo methylation of DNA sequences are unknown. A "natural" gene transfer system, the introduction of unmethylated mitochondrial DNA into the nuclear genome, is being used to investigate sequence requirements for de novo methylation.